It’s not clear how water retention would help with the needed flushing.

Hypothetically, more water retention would mean that the fluid being flushed is less concentrated, and if the flushing mechanism is triggered by a certain concentration level then it'll happen less frequently.

Hard to imagine that it would be worth more than a few percent though.

Less flushing sounds like it would also worsen the sleep deprivation, even if it reduces the momentary lapses.